Tracking Apparatus and Method

ABSTRACT

A tracking apparatus includes a photosensor. The apparatus includes only a single, physically compact, optical pattern emitting base station. The apparatus includes a computer that tracks the photosensor to sub-millimeter accuracy using the optical pattern emitted by the base station. Alternatively, the computer determines angular position of the photosensor relative to the base station to a finer resolution than the size of an aperture of the photosensor from the light emitted by the base station. A method for tracking.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a nonprovisional of U.S. provisional application Ser. No.62/290,183 filed Feb. 2, 2016, incorporated by reference herein.

FIELD OF THE INVENTION

The present invention is related to tracking an object with aphotosensor on the object using a single base station that emits lightthat is received by the photosensor. (As used herein, references to the“present invention” or “invention” relate to exemplary embodiments andnot necessarily to every embodiment encompassed by the appendedclaims.). More specifically, the present invention is related totracking an object with a photosensor on the object using a single basestation that emits light that is received by the photosensor where thelight gas macro-patterns and micro-patters that are used for identifyingthe position of the photosensor and thus the object.

BACKGROUND OF THE INVENTION

This section is intended to introduce the reader to various aspects ofthe art that may be related to various aspects of the present invention.The following discussion is intended to provide information tofacilitate a better understanding of the present invention. Accordingly,it should be understood that statements in the following discussion areto be read in this light, and not as admissions of prior art.

Systems that optically track the position of a target have many uses,including inventory control, security, movement analysis and virtualreality. Some of these systems, such as the Valve/HTC Lighthouse [Deyle2015] and the Instant Replay system of Raskar et al [Raskar 2006] usesmall photosensors on the object to be tracked. One or more fixedlocation base stations emit time-varying patterns of angularlystructured light into the scene. The resulting time-varying lightintensity measured at each photosensor is used to calculate the angle ofthat photosensor with respect to the origin point of that transmittedpattern.

Two orthogonally oriented patterns emitted sequentially from the samebase station can then be used to compute the solid angle of thephotosensor from that base station. Triangulation to compute the threedimensional position of the photosensor in the scene can then beeffected through the use of two or more base stations in differentlocations, or by placing multiple photo-sensors in different knownlocations on a shared rigid body to be tracked.

These two systems have the benefit that the target photosensors aresmall, and therefore can be placed unobtrusively on objects to betracked, or placed in multiple locations on non-rigid objects, such asthe bodies or clothing of people to be tracked. For example, eithersystem can be used to track the position of a wand that is drawing inthe air in a virtual reality simulation. In this application aphotosensor can be placed at the tip of the wand to be tracked.

In another example, either system can be used to track the position andorientation of a virtual reality head mounted display (HMD). In thisapplication a number of photosensors can be placed on different knownlocations of the HMD. Once the location of each photosensor has beendetermined, then a “best fit” rigid body can be readily computed fromthe measured locations of these individual photosensors.

Each of these two systems suffers from practical limitations inmeasurable angular resolution, due to limitations on practical opticalresolution in different parts of the system. The Lighthouse systemrequires a linear sweep of the scene by a scanning laser line for eachangular dimension to be measured. The specific moment in time duringthis sweep when the scanning line impinges on a photosensor target isused to compute the angular position of that photosensor in thedimension of the sweep, with respect to the emitting base station.

This approach has a resolution limitation due to the fact that the sweepneeds to be fast enough for real-time tracking. The Lighthouse systemdoes a complete measurement 60 times per second. This requires foursweeps (one horizontal followed by one vertical for each of two basestations).

This constraint puts a large burden on the timing circuitry on thereceiving end that converts detection time to position. Practically thislimitation results in a final positional accuracy within the scene thatcannot be smaller than about 10 millimeters, for targets that are on theorder of two meters away from the base stations.

The Instant Replay system projects a discrete Gray code pattern out tothe scene in ten sequential steps (one per binary bit), to determineangular resolution in each dimension. This system has the advantage overthe Lighthouse that angular positional resolution is exponential intime: Only n discrete time steps are required to measure 2̂n discreteangular positions.

In practice, Instant Replay uses 10 sequentially projected bit patternsto encode 10̂2, or 1024, angular positions. Yet this nominal highestangular resolution is achieved only if the receiving aperture of thephotosensor is smaller than the pattern detail size at the finestresolution (lowest order bit) of the projected Gray code pattern.

The measurable angular resolution is therefore limited by the size ofthe optical aperture formed by the photosensor, which in a practicalimplementation needs to be large enough to gather sufficient light toguarantee an acceptably high signal to noise ratio. Pattern detail thatis smaller than the size of the receiving aperture cannot be accuratelydetected.

BRIEF SUMMARY OF THE INVENTION

The present invention pertains to a tracking apparatus. The apparatuscomprises a photosensor. The apparatus comprises only a single,physically compact, optical pattern emitting base station. The apparatuscomprises a computer that tracks the photosensor to sub-millimeteraccuracy using optical pattern emitted by the base station.

The present invention pertains to a tracking apparatus. The apparatuscomprises a photosensor having an aperature. The apparatus comprises alight emitting base station. The apparatus comprises a computer thatdetermines angular position of the photosensor relative to the basestation to a finer resolution than the size of the aperture of thephotosensor from the light emitted by the base station.

The present invention pertains to a method for tracking. The methodcomprises the steps of emitting an optical pattern with only a single,physically compact base station. There is the step of tracking aphotosensor with a computer to sub-millimeter accuracy using the opticalpattern emitted by the base station.

The present invention pertains to a method for tracking. The methodcomprises the steps of emitting light from a base station. There is thestep of determining angular position of a photo-sensor having anaperture relative to the base station with a computer to a finerresolution than the size of the aperture of the photosensor from thelight emitted by the base station.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

In the accompanying drawings, the preferred embodiment of the inventionand preferred methods of practicing the invention are illustrated inwhich:

FIG. 1 shows depth position is more uncertain than horizontal position.

FIG. 2 is an inventory of physical parts.

FIG. 3 shows the structure of the rotating pattern disk.

FIGS. 4A-4C show a series of micro-patterns.

FIG. 5 shows the photosensor changing position.

FIGS. 6A-6D show a series of patterns with both macro- andmicro-measurement.

FIG. 7 shows light from laser passes through a pattern generator,sending an angularly diffracted stripe pattern into the room.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like reference numerals refer tosimilar or identical parts throughout the several views, and morespecifically to FIG. 2 thereof, there is shown a tracking apparatus 100.The apparatus 100 comprises a photosensor 5. The apparatus 100 comprisesonly a single, physically compact, optical pattern emitting base station10. The apparatus 100 comprises a computer 1 that tracks the photosensor5 to sub-millimeter accuracy using the optical pattern emitted by thebase station 10.

The base station 10 may include a disk 2 with a motor 3 that rotates thedisk 2. The disk 2 may have an outer edge 12 with a sequence oftransmissive fraction patterns. There may be between 40 and 240diffraction patterns. The base station 10 may include at least one laser4 behind the rotating patterns of the disk 2 which emits light throughthe rotating patterns. The base station 10 may include 4 lasers placedat regular angular locations of 0, π/2, π and 3 π/2 with respect to thedisk 2.

The diffraction patterns may include a first set of macro-patterns 14for macro-measurement disposed one immediately after another, and afirst set of micro-patterns 16 far micro-measurement disposed oneimmediately after another, each micro-pattern 16 smaller than everymacro-pattern 14, as shown in FIG. 3. Each pattern may cause collimatedlaser 4 light which impinges upon it from behind to scatter in astructured stripe 22 pattern. Each emitted stripe 22, as shown in FIG.5, may run perpendicular to a radius of the disk 2, and the stripe 22pattern varies in the disk's radial direction. At least one of thediffraction patterns may be a synchronization pattern 18. There may be ablanking interval 20 between two diffraction patterns. A micro-positionof the sensor may be determined by fitting a sine wave to a receivedtime-varying micro-measurement associated with the micro-patterns 16,where a phase of the sine wave determines micro disposition of thephotosensor 5, with the phase shifting linearly with position of thephotosensor 5.

The present invention pertains to a tracking apparatus 100. Theapparatus 100 comprises a photosensor 5 having an aperature 75. Theapparatus 100 comprises a light emitting base station 10. The apparatus100 comprises a computer 1 that determines angular position of thephotosensor 5 relative to the base station 10 to a finer resolution thanthe size of the aperture of the photosensor 5 from the light emitted bythe base station 10.

The present invention pertains to a method for tracking. The methodcomprises the steps of emitting an optical pattern 77, as shown in FIG.7, with only a single, physically compact base station 10. There is thestep of tracking a photosensor 5 with a computer 1 to sub-millimeteraccuracy using the optical pattern emitted by the base station 10.

The present invention pertains to a method for tracking. The methodcomprises the steps of emitting light from a base station 10. There isthe step of determining angular position of a photosensor 5 having anaperture 75 relative to the base station 10 with a computer 1 to a finerresolution than the size of the aperture 75 of the photosensor 5 fromthe light emitted by the base station 10.

The current invention extends the paradigm of Instant Replay so that theangular resolution of measurement along each dimension can be an orderof magnitude finer than the finest resolution of the projected patternand can also an order of magnitude smaller than the size of thereceiving aperture.

Using two such angular sensors oriented orthogonally to each other, asis taught by [RASKAR 2006], a solid angular position from each basestation 10 can be measured that is an order of magnitude more accuratethan Instant Replay in each angular dimension. Then, as in theLighthouse and Instant Replay systems, by deploying two or more patternprojectors which have been placed at different locations, depth can becomputed from stereo disparity between these two pattern emitting basestations 10.

The order of magnitude increase in accuracy achieved by the currentinvention allows the distance between the base stations 10 to be anorder of magnitude smaller, given the same accuracy of measureddistance, than is the case for either the Lighthouse or Instant Replaysystems.

Consider, for example, the case in which a wand which has been outfittedwith a single photosensor 5 at its far end is being tracked by anInstant Replay system. When the wand is three meters away from the basestation 10, the Instant Replay system, which has a 0.466 radianhorizontal field of view, will cover an area 1.42 meters wide, since2×tan(0.466/2)×3 m=1.42 m. Within that span, it will be able todistinguish 1024 distinct locations, which gives it a horizontalpositional accuracy to within about 1.42 mm at that distance. Thecurrent invention, in contrast, will have horizontal positional accuracyat that distance to about 0.14 mm.

Consider the case where two Instant Replay systems are side by side,separated by 265 mm, which is the stereo separation distance of the twohorizontally displaced laser 4 emitters of one embodiment of the currentinvention described below. As can be seen in FIG. 1, divergent stripe 22patterns emerging from two base stations (1) impinge upon a photosensor5 (2) with a horizontal resolution (3). Because the distance from thebase stations to the target photosensor 5 is greater than the separationdistance between the two base stations, the resolvable depth at thephotosensor 5 will be more uncertain than the resolvable horizontalposition. In the case de-scribed, depth uncertainty will be about 16.1mm, which is unacceptably large for many applications, such as drawingin the air and measurement of the position of 3D objects. The currentinvention, in contrast, will have a depth accuracy for the same wandpoint to within about 1.6 mm, which is sufficiently accurate in practicefor both of those applications. This is merely a representative example.The stereo separation could be, for example, 100 mm or 500 mm.

This increased positional accuracy allows full 3D tracking to beaffected using a single compact base station 10, rather than requiringbase stations to be separated far apart from each other. This compactform factor also allows the two pattern emitting units to be rigidlypositioned with respect to each other, thereby obviating the need forcalibration between the two pattern emitting units.

The physical parts consist of:

1. Computer 1

2. Pattern disk 2

3. Motor 3

4. Lasers 4

5. Photosensors

6. Electronics for photosensor 5

In one embodiment, under the control of computer (1), a disk (2) isrotated at 7200 rpm (120 rotations per second). This rotation can beeffected by a standard motor 3 of the kind commonly found in magneticdisk drives (3). Near its outer edge 12 the rotating disk 2 contains asequence of transmissive diffraction patterns. This is merely arepresentative example. Rotation could be 2000 rpm or 25000 rpm oranywhere in between and the method of operation will remain the same.

A disk 267 mm in diameter, and therefore with a circumference of 840 mm,will rotate past any fixed point at a linear velocity of 100800 mm persecond (120×840 mm). If each successive diffraction pattern occupies 10mm along the periphery of the disk 2, then 10080 discrete patterns persecond will rotate past any fixed point. This is sufficient to encode 80discrete patterns around the disk 2 circumference, plus four periodicblanking intervals 20 each rotation to maintain synchronization. Notethat any of these parameters can be varied. 80 patterns around the disk2 circumference is merely representative. 40 or 240 patterns or anyappropriate number In between would also be appropriate values.

In one embodiment, four solid state lasers (4) are placed at the regularangular locations 0, π/2, π and 3π/2 behind the rotating pattern.

The pattern disk (2) contains 20 successive diffraction patterns plus ablanking interval 20 for synchronization around a contiguous ¼ of itsperiphery. The sequence of patterns consists of 20 discrete patterns: 10patterns for the macro-measurement, after [Raskar 2006], as well as 10patterns for the micro-measurement, followed by a blanking interval 20.Each pattern causes collimated laser light which impinges upon it frombehind to scatter in a structured stripe 22 pattern. In one embodimentthis can be done using standard diffraction technology such as isemployed in the Microsoft Kinect [Martinez 2014]. Each emitted stripe 22runs perpendicular to the radius of the disk 2, and the stripe 22pattern therefore varies in the disk's radial direction.

FIG. 3 shows the structure of this disk 2 in greater detail. Therotating disk 2 contains 21 diffraction patterns sequentially arrangedaround ¼ of its circumference. The first pattern is a synchronizationpattern 18. This is followed by 10 macro-patterns 14, between a firstmacro-pattern 33 and a last macro-pattern 44. Finally there are 10micro-patterns 16, between a first micro-pattern 55 and a lastmicro-pattern 66.

As the disk 2 rotates, this sequence of 21 patterns moves in turn pasteach of the four lasers 4. One complete rotation of the pattern disk 2therefore effects a full cycle of 84 projected patterns, resulting intwo horizontal pattern sequences separated by a 267.4 mm horizontalstereo base line (originating from the lasers 4 located at 0 and πaround the disk 2) and two vertical pattern sequences separated by a267.4 mm vertical stereo base line (originating from the lasers 4located at π/2 and 3π/2 around the disk 2). FIG. 7 and the textaccompanying it below describes in more detail how the sequence ofdiffraction pattern generators moving past each laser 4 will emitangularly varying stripe 22 patterns, the light from which will then goto photosensors in the 3D space.

For the micro-measurement, the time varying phase offset canalternatively be projected as a stripe 22 pattern that shifts phasecontinuously over time, rather than as a sequence of discretelyphase-shifted stripe 22 patterns. This alternative permits thephotosensor receiver unit (5), (6) to use analog logic to fit a moreprecise phase shifted sinusoidal wave than might be possible with asequence of discrete digital patterns. The 267.4 mm disk 2 diameter ismerely representative. A disk 2 providing stereo separation of 100 mm or500 mm would also be appropriate.

Step by Step Internal Operation in Best Embodiment

To the user, the operation is as follows:

1. Move the photosensor 5 to a location in the room.

2. Read where the photosensor 5 is located.

Step by Step Internal Operation in Best Embodiment

As the spinning set of patterns rotates, it makes a succession ofdiffraction patterns appear in front of the laser 4. In the first“macro-measurement” portion of the pattern sequence, the macro-positionis transmitted as a sequence of Gray code bit patterns, as in [RASKAR2006]. The strength of the laser 4 can be 60 mW, as in the MicrosoftKinect [Martinez 2014], or can be a different strength, such as 20 mW or120 mW. All things being equal, the more powerful the laser 4, thehigher will be the signal to noise ratio. Yet a laser 4 which isextremely powerful might be less sale. 60 mW is a useful practicalcompromise between these extremes.

In the second “micro-measurement” portion of the pattern sequence, astriped pattern, with each successive on/off stripe 22 pair the sameangular size as two units in the bit sequence of the macro-measurementGray code bit pattern, shifted laterally as a linear function, offractional time within this micro-measurement. A photosensor 5 in agiven location along this angular dimension will receive the differentphases of the pattern as a roughly sinusoidal time-varying value.

The micro-position of the sensor is determined by fitting a sine wave tothe received time-varying micro-measurement. The phase of this sine wavedetermines the micro-position of the photosensor 5 target. This phasewill shift linearly with position of the photosensor 5 target, shiftingby π for each successive unit bit transition in the macro-measurementGray code. This micro-position adjustment is added to the measuredmacro-position to produce a final higher resolution angular positionalong this dimension.

This two-step process is then repeated to measure angular position alongthe orthogonal angular dimension, to obtain a high resolution solidangle position of the photosensor 5 with respect to this base station10. As in [Deyle 2015] and [Raskar 2006], the use of two or more offsetbase stations, which in one embodiment can be multiple known laser 4emitter locations around a spinning disk 2, as well as by combining theresults measured at multiple photosensor 5 locations that are at a knownphysical offset from each other, can then be used to construct a 3Dposition for each photosensor 5, as well as 3D orientation for any rigidbody that contains photosensors at multiple locations.

The apparatus 100 provides for

(1) Ability to track the position of a photosensor 5 target tosub-millimeter accuracy using a single, physically compact, opticalpattern emitting base station 10.

(2) Ability to determine angular position of a photosensor 5 withrespect to a light emitting base station 10 to a finer resolution thanthe size of the aperture of the photosensor 5. The aperture size, forexample, can be 0.5 mm to 5 mm.

FIGS. 4A-4C show a series of micro-patterns that are used to compute themicro-position of the photosensor 5 on or with an object from the phaseof a sine wave fitted to the micro measurements.

FIGS. 6A-6D show the photosensor 5 changing position by one unit, withboth micro and macro measurements.

The spinning disk (1) is positioned in front of four lasers (2). As thedisk 2 spins, each of its sequence of light diffracting patterngenerators (3) becomes positioned, in its turn, in front of one of thelasers 4. The light from that laser 4 is then diffracted by the patterngenerator into an angularly deflected stripe 22 pattern (4). Thisangularly patterned light will eventually impinge upon the apertures ofphotosensors in the room.

REFERENCES, all of which are incorporated by reference, herein:

Deyle, T: Valve's “Lighthouse” Tracking System May Be Big News forRobotics, 1015.

Martinez, M., Stiefelhagen, R., Kinect Unbiased, Image Processing (ICIP)IEEE International Conference on, 5791-5795, 2014.

Raskar et al: System and method for sensing geometric and photometricattributes of a scene with multiplexed illumination and solid statesoptical devices U.S. Pat. No. 8,009,192 B2, 2006.

Although the invention has been described in detail in the foregoingembodiments for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be described by thefollowing claims.

1. A tracking apparatus comprising: a photosensor; only a single,physically compact, optical pattern emitting base station; and acomputer that tracks the photosensor to sub-millimeter accuracy usingthe optical pattern emitted by the base station.
 2. The apparatus ofclaim 1 wherein the base station includes a disk with a motor thatrotates the disk.
 3. The apparatus of claim 2 wherein the disk has anouter edge with a sequence of transmissive fraction patterns.
 4. Theapparatus of claim 3 wherein there are between 40 and 240 diffractionpatterns.
 5. The apparatus of claim 4 wherein the base station includesat least one laser behind the rotating patterns of the disk which emitslight through the rotating patterns.
 6. The apparatus of claim 5 whereinthe base station includes 4 lasers placed at regular angular locationsof 0, π/2, π and 3 π/2 with respect to the disk.
 7. The apparatus ofclaim 6 wherein the diffraction patterns include a first set ofmacro-patterns for macro-measurement disposed one immediately afteranother, and a first set of micro-patterns for micro-measurementdisposed one immediately after another, each micro-pattern smaller thanevery macro-pattern.
 8. The apparatus of claim 7 wherein each patterncauses collimated laser light which impinges upon it from behind toscatter in a structured stripe pattern.
 9. The apparatus of claim 8wherein each emitted stripe runs perpendicular to a radius of the disk,and the stripe pattern varies in the disk's radial direction.
 10. Theapparatus of claim 9 wherein at least one of the diffraction patterns isa synchronization pattern.
 11. The apparatus of claim 10 wherein thereis a blanking interval between two diffraction patterns.
 12. Theapparatus of claim 11 wherein a micro-position of the photosensor isdetermined by fitting a sine wave to a received time-varyingmicro-measurement associated with the micro-patterns, where a phase ofthe sine wave determines the micro disposition of the photosensor, withthe phase shifting linearly with position of the photosensor.
 13. Atracking apparatus comprising: a photosenser having an aperature; alight emitting base station; and a computer that determines angularposition of the photosensor relative to the base station to a finerresolution than the size of the aperture of the photosensor from thelight emitted by the base station.
 14. A method tracking comprising thesteps of: a photosensor; emitting an optical pattern with only a single,physically compact base station, and tracking a photosensor with acomputer to sub-millimeter accuracy using the optical pattern emitted bythe base station.
 15. A method for tracking comprising the steps ofemitting light from a base station; and determining angular position ofa photosensor having an aperture relative to the base station with acomputer to a finer resolution than the size of the aperture of thephotosensor from the light emitted by the base station.